1. Field of the Invention
The present invention relates to a zirconia-based composite ceramic sintered compact colored by a neodymium ion solution or a neodymium complex solution, and a method producing the same.
2. Description of the Conventional Art
Compared with metal materials or plastic materials, ceramic materials are excellent in hardness, wear resistance, heat resistance, corrosion resistance, and the like. In a dental field, ceramics materials, such as an alumina sintered compact being chemically stable, a zirconia sintered compact being excellent in strength and toughness, an alumina/zirconia composite sintered compact, and the like, are formed and processed to be a dental prosthesis by using CAD/CAM, and used clinically.
Zirconia has a monoclinic system at a room temperature, and a crystal structure of zirconia is phase-transited to a tetragonal system and then to a cubic system as increasing the temperature. Since the phase transition accompanies changing in volume, the sintered compact comes to be broken by repeatedly heating and cooling. Therefore, partially stabilized zirconia has been widely used. The partially stabilized zirconia is made by solid-dissolving rare-earth oxide, such as yttrium oxide, cerium oxide or the like, in zirconia as a stabilizer and forming oxygen vacancies in the crystal structure. The partially stabilized zirconia can suppress breaking of the sintered compact due to heating and cooling.
The yttrium-based partially stabilized zirconia in which yttrium oxide is solid-dissolved as a stabilizer is suitable for a dental prosthesis because of having a white color tone. However, since the tetragonal system which is a metastable phase is phase-transited to the monoclinic system at a comparatively low temperature (200 to 300° C.), there is a problem that cracks are generated inside due to volume expansion at the time of transformation so as to decrease strength. Furthermore, the phase transition is generated even at a low temperature of about 37° C. in such a wet environment as inside of an oral cavity, so that it has been pointed out that the surface is roughed or the strength decreases.
Further, the ceria-based partially stabilized zirconia in which cerium oxide is solid-dissolved as a stabilizer hardly deteriorates at low temperature and has high strength and toughness. As the ceria-based partially stabilized zirconia, a zirconia-alumina composite ceramic material consists of a first phase and a second phase has been disclosed (for example, refer to Japanese Patent Application Laid-Open Nos. 2004-051481, 2005-097094, 2005-306726, and 2006-271435). In the zirconia-alumina composite ceramic material, the first phase includes a ZrO2 particle including ceria of 10 to 12 mol % as a stabilizer and having an average particle diameter of 0.1 to 1 μm. The second phase includes an alumina particle deameter of 0.1 to 0.5 μm. However, in a case that the cerium oxide is used as a stabilizer, since the cerium oxide itself has a yellowish appearance, the color tone after sintering also comes to be yellowish a little. Therefore, when it is used as a dental prosthesis whose color is basically white, there arises a problem that the aesthetic property is poor.
As a conventional coloring method of zirconia, a method of adding metal oxide as a colorant, such as chromium oxide, nickel oxide, cobalt oxide, vanadium oxide or the like, according to a desired color tone, mixing them and sintering the mixture has been widely used (for example, refer to Japanese Patent Application Laid-Open Nos. 2005-306678 and 2004-059374). However, in this method, since thermal expansion coefficient and elastic modulus of the metal oxide used as a colorant are different from those of zirconia, a thermal stress and a residual stress are generated on a contact interface between a zirconia particle and the metal oxide at a time of sintering, so that pores and cracks are generated inside crystals of the obtained zirconia sintered compact. So, there is a problem that toughness and corrosion resistance are lacked, decreasing the strength is caused, or the like. Furthermore, since an operation of adding the metal oxide to the zirconia powder and mixing them are carried out in a powdery state, homogeneous dispersing is hard and color unevenness is easily caused.
Thus, as a method capable of coloring homogeneously and accurately by an easy process while maintaining mechanical and physical properties, a method of dipping a zirconia molded product in a liquid solution obtained by dissolving a color metal, impregnating the coloring metal into the zirconia formed product, and sintering them has been disclosed (for example, refer to Japanese Translation of PCT International Application Nos. 2002-536280 and 2010-534245). Japanese Translation of PCT International Application No. 2010-534245 discloses a method of using a solution containing at least one kind of salts or complexes of rare earth elements or subgroup elements having a specific concentration, and coloring translucent ceramics in a porous or absorbent state.
However, since the ceria-based partially stabilized zirconia is strongly yellowish. So, even when the formed product is dipped in the conventional a liquid solution obtained by dissolving a color metal so as to be impregnated with the coloring metal, the formed product is hardly colored to white. Therefore, a ceria-based partially stabilized zirconia material having a color tone suitable to be used as a dental prosthesis has been desired.